Water heater with forced draft air inlet

ABSTRACT

A water heater includes an air intake assembly that includes a blower for providing primary and secondary air to a combustion chamber at pressures above atmospheric pressure. The primary air is mixed with gaseous fuel and the mixture is combusted at a burner in a partially premixed but substantially diffusion flame having an envelope. Combustion of the mixture is completed within the envelope in the presence of secondary air at elevated pressure. A 24 V controller provides power to a user interface and a powered anode in the tank. The controller receives input from a pressure sensor in the air intake assembly, a flame sensor in the combustion chamber, and a flammable vapor sensor outside the water heater. The controller controls operation of the blower and a gas valve. A flue and baffle arrangement in the tank causes products of combustion from the burner to lose pressure and vent at near atmospheric pressure at the top of the flue. An air distributor plate creates a substantially uniform distribution of pressurized secondary air within the combustion chamber.

This application claims priority under 35 U.S.C. 120 to U.S. patentapplication Ser. No. 11/865,378 filed Oct. 1, 2007, the entire contentsof which are incorporated herein by reference.

BACKGROUND

The present invention relates to a water heater having a forced draftair inlet.

SUMMARY

In one embodiment, the invention provides a water heater comprising: atank for water to be heated; a powered anode extending into the tank andcreating an electrical current to reduce corrosion of the tank; acombustion chamber; an exhaust structure; a flue in the tankcommunicating between the combustion chamber and the exhaust structure;a burner in the combustion chamber operable to burn a mixture of primarycombustion air with gaseous fuel in a partially premixed butsubstantially diffusion flame having an envelope, such that combustionof the mixture is completed at the diffusion flame envelope in thepresence of secondary air to produce products of combustion, theproducts of combustion flowing through the flue to the exhaust structureto heat the water in the tank; a centrifugal blower forcing primary andsecondary air into the combustion chamber at pressure above atmospheric;a gas valve for controlling a supply of gaseous fuel to the burner; auser interface for programming operating parameters of the water heater;and a 24 V controller that provides power to the user interface andpowered anode, and that controls operation of the blower and gas valve.

In some embodiments, the water heater may include a pressure sensoroperatively interconnected with the 24 V controller; wherein thepressure sensor generates a signal in response to sensing that thepressure of air downstream of the blower is below a minimum threshold;and wherein the 24 V controller closes the gas valve in response toreceiving the signal from the pressure sensor. In some embodiments, thewater heater may include a flammable vapor sensor operativelyinterconnected with the 24 V controller; wherein the flammable vaporsensor generates a signal in response to sensing the presence offlammable vapors outside of the water heater in concentrations above amaximum threshold; and wherein the 24 V controller closes the gas valvein response to receiving the signal from the flammable vapor sensor. Insome embodiments, the water heater may include a pressure sensor sensingthe pressure of air downstream of the blower; and a flammable vaporsensor sensing the presence of flammable vapors outside of the waterheater; wherein the 24 V controller closes the gas valve upon theoccurrence of any of the following: (a) conditions dictated by the userinterface, (b) the pressure sensor sensing air pressure below a minimumthreshold, and (c) the flammable vapor sensor sensing flammable vaporsexternal to the water heater in concentrations above a maximumthreshold.

In another embodiment, the invention provides a water heater comprising:a tank for water to be heated; a combustion chamber; an exhauststructure; a flue in the tank communicating between the combustionchamber and the exhaust structure; a burner in the combustion chamberoperable to burn a mixture of primary combustion air with gaseous fuelin a partially premixed but substantially diffusion flame having anenvelope, such that combustion of the mixture is completed at thediffusion flame envelope in the presence of secondary air to produceproducts of combustion, the products of combustion flowing through theflue to the exhaust structure to heat the water in the tank; an airintake assembly including an air inlet above the combustion chamber anda conduit communicating between the air inlet and the combustionchamber; a centrifugal blower within the air intake assembly operable tosuck air into the air intake assembly through the air inlet and forceprimary and secondary air into the combustion chamber through theconduit at pressure above atmospheric; a gas valve for controlling asupply of gaseous fuel to the burner; a controller that controlsoperation of the blower and gas valve; and a flammable vapor sensorexternal of the combustion chamber and lower than the air inlet, theflammable vapor sensor being operatively interconnected with thecontroller and operable to generate a signal in response to sensing thepresence of flammable vapors outside of the water heater inconcentrations above a maximum threshold; wherein all primary andsecondary combustion air supplied to the combustion chamber flowsthrough the air inlet and conduit; and wherein the controller closes thegas valve in response to receiving the signal from the flammable vaporsensor. In some embodiments, the flammable vapor sensor is lower than atleast one of the combustion chamber and burner.

In another embodiment, the invention provides a water heater comprising:a tank for water to be heated; a combustion chamber; an exhauststructure; a flue in the tank communicating between the combustionchamber and the exhaust structure; a burner in the combustion chamberoperable to burn a mixture of primary combustion air with gaseous fuelin a partially premixed but substantially diffusion flame having anenvelope, such that combustion of the mixture is completed at thediffusion flame envelope in the presence of secondary air to produceproducts of combustion, the products of combustion flowing through theflue to the exhaust structure to heat the water in the tank; an airintake assembly including an air inlet above the combustion chamber anda conduit communicating between the air inlet and the combustionchamber; a centrifugal blower within the air intake assembly operable tosuck air into the air intake assembly through the air inlet and forceprimary and secondary air into the combustion chamber through theconduit at pressure above atmospheric; a gas valve for controlling asupply of gaseous fuel to the burner; a controller that controlsoperation of the blower and gas valve; and a baffle in the flue thatrestricts flow sufficiently to reduce the pressure of the products ofcombustion to near atmospheric upon flowing out of the flue into theexhaust structure. In some embodiments, the blower is an axial-intake,centrifugal blower. In some embodiments, the exhaust structure is acategory I vent structure.

In another embodiment, the invention provides a water heater comprising:a tank for water to be heated; a combustion chamber; an exhauststructure; a flue in the tank communicating between the combustionchamber and the exhaust structure; a burner in the combustion chamberoperable to burn a mixture of primary combustion air with gaseous fuelin a partially premixed but substantially diffusion flame having anenvelope, such that combustion of the mixture is completed at thediffusion flame envelope in the presence of secondary air to produceproducts of combustion, the products of combustion flowing through theflue to the exhaust structure to heat the water in the tank; an airintake assembly including an air inlet above the combustion chamber anda conduit communicating between the air inlet and the combustionchamber; a centrifugal blower within the air intake assembly operable tosuck air into the air intake assembly through the air inlet and forceprimary and secondary air into the combustion chamber through theconduit at pressure above atmospheric; a gas valve for controlling asupply of gaseous fuel to the burner; and a controller that controlsoperation of the blower and gas valve; wherein the air intake assemblyincludes an interior space, all primary and secondary air being providedto the combustion chamber flowing through the interior space; andwherein the blower is mounted within the interior space of the airintake assembly.

In some embodiments, the blower is an axial-intake, centrifugal blower.In some embodiments, the air intake assembly includes a longitudinalextent; and the air intake assembly includes a two-piece constructiondivided along the longitudinal extent of the air intake assembly. Insome embodiments, the interior space of the air intake assembly isnon-cylindrical and has an equivalent hydraulic diameter of a four inchinner diameter tube. In some embodiments, at least a portion of theinterior space of the air intake assembly is non-cylindrical toaccommodate mounting the blower in the interior space; and thenon-cylindrical portion of the interior space defines a minor dimensionand a major dimension that is perpendicular to the minor dimension andat least twice the minor dimension. In some embodiments, the air intakeassembly includes a partition that divides the interior space into aninlet side communicating with ambient air and an outlet sidecommunicating with the combustion chamber, the partition including awindow; and the blower is within the inlet side of the interior spaceand forces primary and secondary air into the outlet side of theinterior space through the window in the partition. In some embodiments,the air intake assembly includes a louvered opening communicatingbetween the inlet side and ambient air; and all air sucked into theinlet side of the interior space by the blower flows through thelouvered opening. In some embodiments, the water heater furthercomprises a pressure sensor communicating with the interior space, thepressure sensor operable to disable the gas valve to cut off the supplyof gaseous fuel to the burner when pressure within the interior spacedrops below a minimum threshold. In some embodiments, the air intakeassembly includes an exterior surface, a sensor mounting cavity in theexterior surface and a wire routing channel in the exterior surface; theair intake assembly further includes a hole communicating between thesensor mounting cavity and the interior space; the pressure sensor ismounted within the sensor mounting cavity and communicates with theinterior space through the hole; and the blower includes a power cordthat is received in the wire routing channel.

In another embodiment, the invention provides a water heater comprising:a tank for water to be heated; a combustion chamber; an exhauststructure; a flue in the tank communicating between the combustionchamber and the exhaust structure; a burner in the combustion chamberoperable to burn a mixture of primary combustion air with gaseous fuelin a partially premixed but substantially diffusion flame having anenvelope, such that combustion of the mixture is completed at thediffusion flame envelope in the presence of secondary air to produceproducts of combustion, the products of combustion flowing through theflue to the exhaust structure to heat the water in the tank; an airintake assembly including an air inlet above the combustion chamber anda conduit communicating between the air inlet and the combustionchamber; a centrifugal blower within the air intake assembly operable tosuck air into the air intake assembly through the air inlet and forceprimary and secondary air into the combustion chamber through theconduit at pressure above atmospheric; a gas valve for controlling asupply of gaseous fuel to the burner; a controller that controlsoperation of the blower and gas valve; an air distributor plate having agenerally horizontal top surface defining a plurality of edges, and abottom surface at least partially defining a secondary air distributionspace and a primary air plenum; wherein all primary and secondary airflows into the respective primary air plenum and secondary airdistribution space; wherein the air distributor plate defines a primaryair opening for the provision of primary air from the primary air plenumto the burner; and wherein the air distributor plate includes aplurality of secondary air openings for substantially axisymmetricdistribution of secondary air around the air distributor plate.

In some embodiments, the air distributor plate has first, second, third,fourth, fifth, and sixth edges; wherein the air distributor plate hasgenerally vertical surfaces extending from the first, second, third,fourth, and fifth edges to at least partially define the secondary airdistribution space; wherein the generally vertical surfaces define thesecondary air openings; and wherein the substantially axisymmetricdistribution includes secondary air flow rates through the pluralitysecondary air openings with standard deviation less than 0.88 fromaverage secondary air flow rate. In some embodiments, the standarddeviation is less than 0.10. In some embodiments, the standard deviationis about 0.08. In some embodiments, the water heater further comprisesan air diverter secured to an edge of the air distributor plate todirect all primary and secondary combustion air from an air intake tothe respective primary air plenum and secondary air distribution space.In some embodiments, the water heater further comprises a plenum panmounted to a bottom surface of air distributor plate to at leastpartially define the primary air plenum. In some embodiments, the plenumpan includes a plurality of tabs; the distributor plate includes aplurality of slots through which tabs extend; and the tabs are bentagainst the top surface of distributor plate to secure the plenum pan tothe bottom surface of the distributor plate. In some embodiments, thedistributor plate includes an integral first locating member, anintegral burner locating member, an integral manifold pocket, and anintegral manifold clearance indentation; wherein a portion of the burnermates with the burner locating member; the water heater furthercomprising a gas manifold for the provision of gaseous fuel from the gasvalve to the burner, the manifold extending into the manifold clearanceindentation during installation, and the manifold extending into themanifold pocket when installed; and a condensation tray including afirst mating portion that mates with the first locating member, and asecond mating portion that extends around the manifold pocket. In someembodiments, the distributor plate includes first and second locatingmembers; the water heater further comprising a condensation tray havingfirst and second clocking points that mate with the respective first andsecond locating members; wherein the condensation tray is secured to thedistributor plate with a single threaded fastener in combination withmating of the first and second clocking points with the first and secondlocating members. In some embodiments, the water heater furthercomprises a condensation tray mounted in the combustion chamber; whereinthe burner includes a condensate drain that directs condensation to thecondensation tray. In some embodiments, the condensation tray has acontainment capacity at least equal to the volume of condensatepredicted during heavy condensation cold start of the water heater. Insome embodiments, the condensation tray is dimensioned to cause asufficient surface area of condensate in the condensation tray to beexposed to heat in the combustion chamber to result in total evaporationof the condensate upon the water heater reaching steady-state combustionand normal operation.

Other aspects of the invention will become apparent by consideration ofthe detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a water heater embodying the presentinvention.

FIG. 2 is an exploded view of the water heater.

FIG. 3 is a cross-section view of the water heater taken along line 3-3in FIG. 1.

FIG. 4 is an illustration of the control system and wiring of the waterheater.

FIG. 5 an exploded view of an air intake assembly of the water heaterfrom a first perspective.

FIG. 6 is an exploded view of the air intake assembly from a secondperspective.

FIG. 7 is a cross-section view of the air intake assembly.

FIG. 8 is an exploded view of the combustion chamber assembly of thewater heater.

FIG. 9 is an exploded view of the combustion chamber assembly from asecond perspective.

FIG. 10 is a perspective view of a partially assembled combustionchamber assembly with arrows indicating the flow of primary andsecondary air.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways.

FIGS. 1-3 illustrate a water heater 10 that includes a tank 15 in whichwater is heated and stored, a combustion chamber assembly 20 supportingthe tank 15, an air intake assembly 25 through which combustion air isprovided to the combustion chamber assembly 20, high temperatureinsulation 30 surrounding the combustion chamber assembly 20, a foam dam32 above the high temperature insulation 30, and a jacket 35 surroundingthe tank 15, high temperature insulation 30, and foam dam 32.Foamed-in-place insulation is introduced into an annual space definedbetween the jacket 35 and tank 15, above the foam dam 32.

The tank 15 includes a flue 40, a baffle 45 in the flue 40, an inletspud 50, an outlet spud 55, an anode spud 60 or anode hole, atemperature probe hole 65, a drain valve 70, and a T&P valve 75 (i.e.,temperature and pressure valve 75). Hot products of combustion createdin the combustion chamber assembly 20 flow up from the combustionassembly through the flue 40. As the products of combustion flow throughthe flue 40, heat is transferred from the products of combustion to theflue 40 wall and then to the water surrounding the flue 40. The baffle45 restricts the flow of products of combustion through the flue 40,which increases the time during which the products of combustion dwellwithin the flue 40. Generally speaking, an increase in the dwell timealso increases the amount of heat transferred from the products ofcombustion to the water in the tank 15 through the flue 40 wall. Also,the pressure of the products of combustion drops as the products ofcombustion flow through the restricted flow path of the flue 40 andbaffle 45 assembly.

There are many types, styles and designs for baffles, and the baffle 45may be removable from the flue 40 or permanently fixed within the flue40 (as with metallurgical bonding such as brazing or welding, or withmechanical fasteners). In other embodiments, the baffle 45 may beintegrally formed with the flue 40 wall. As will be discussed in moredetail below, the baffle 45 restricts the flow of products of combustionsuch that the products of combustion are at atmospheric pressure or nearatmospheric pressure when they flow out of the flue 40. As used in thisdisclosure, “near atmospheric” means a pressure of exhaust that iswithin a range for which a natural draft (e.g., Category I) ventstructure or exhaust structure (as illustrated at 77) is suitable, evenif such pressure of exhaust is above atmospheric pressure.

A cold water pipe 80 is threadedly interconnected to the inlet spud 50and a hot water pipe 85 is threadedly interconnected to the outlet spud55. A dip tube 90 is threaded or otherwise fit within the inlet spud 50.As hot water is drawn from the tank 15 through the hot water pipe 85,cold water flows into the bottom of the tank 15 through the cold waterpipe 80 and dip tube 90. A powered anode 95 (FIG. 4) is threaded orotherwise secured into the anode spud 60. The powered anode 95 generatescurrent which reduces the rate of tank 15 corrosion or eliminates tank15 corrosion altogether. The drain valve 70 permits draining of waterfrom the tank 15 during servicing, and the T&P valve 75 permits pressureto be released from the tank 15 in the event of high pressure within thetank 15 resulting from overheating of the water.

With additional reference to FIG. 4, the water heater 10 also has acontrol system 100, which includes a controller 105, a gas valve 115, auser interface 120, a pressure sensor 125 having a pressure tap 126(FIG. 5), a FV sensor 130 (i.e., a flammable vapor sensor), a hotsurface igniter 135, and a flame sensor 140. The controller 105 in theillustrated embodiment is a 24V controller 105 (i.e., 24 Voltcontroller) which is powered by a 24 Volt power supply which may beprovided, for example, by a transformer 150 that is plugged into astandard voltage outlet. The controller 105 includes a processor forreceiving inputs from the sensors 125, 130, 140 and user interface 120,and monitoring and controlling operation of the water heater 10.

A powered anode wire 160 interconnects the controller 105 and thepowered anode 95 for the provision of power to the powered anode 95.Power and/or communications (as necessary for functionality) between thecontroller 105 and the other control system 100 components are providedby way of a user interface wire 165, a pressure sensor wire 170, a FVsensor wire 175, an igniter wire 180, and a flame sensor wire 185.Wireless communication between the controller 105 and one or more ofthese components is possible for other embodiments.

In the illustrated embodiment, the gas valve 115 and controller 105 areintegrated into a single unit, but in other embodiments the gas valve115 and controller 105 may be separate units with a suitable wiredand/or wireless connection. The gas valve 115 includes a temperatureprobe 190 that extends into the water in the tank 15 through the probehole 65. The gas valve 115 receives a supply of gaseous fuel (e.g.,natural gas) from a source of gaseous fuel, through a hook-up line 200.A supply line 205 delivers the gaseous fuel from the gas valve 115 tothe combustion chamber assembly 20 to create the products of combustiondiscussed above. The user interface 120 permits the operator or user ofthe water heater 10 to program operating parameters, such as targetwater temperature and vacation settings.

The air intake assembly 25 is illustrated in FIGS. 5-7. The air intakeassembly 25 includes a shell 220 having a first piece 225, a secondpiece 230, a partition 235, and a blower 240. The first piece 225includes a pressure sensor cavity 250 formed in its exterior surface(which faces the water heater 10). A central hole 251 in the pressuresensor cavity 250 communicates between the sensor mounting cavity andthe interior space of the air intake assembly 25. The pressure tap 126of the pressure sensor 125 extends through the central hole 251. Thefirst piece 225 also includes a power cord channel 255 or wire routingchannel in its exterior surface. The first piece 225 also defines anelbow 260 which interconnects to the combustion chamber assembly 20. Thefirst piece 225 also includes mounting flanges 265 to accommodatefasteners 270 that secure the air intake assembly 25 to the jacket 35.Mounted to the second piece 230 is air inlet 280, which in theillustrated embodiment is a plate with a louvered window. In otherembodiments, the air inlet 280 may be formed integrally with the secondpiece 230. All combustion air for the water heater 10 flows into thewater heater 10 through the air inlet 280.

To achieve a high quality, attractive shell 220, it is preferable toform the first piece 225 and second piece 230 by injection molding, butthe shell 220 may be alternatively constructed as a single,integrally-formed part through blow molding. The first piece 225 andsecond piece 230 are interconnected with adhesive or another suitablejoining process along a joining line or interface extending generallyalong a longitudinal extent of the air intake assembly 25 (i.e., avertical interface). A gasket 285 is captured between the edges of thefirst piece 225 and second piece 230 at the interface to create asubstantially air tight seal. The surfaces of the first piece 225 andsecond piece 230 that face each other are referred to as their internalsurfaces.

The internal surfaces define an interior space 290 of the air intakeassembly 25. Although the interior space 290 (or the shell 220generally) is non-cylindrical, the geometry of the internal surfaces isdesigned to give the overall air intake assembly 25 a hydraulicfunctionality equivalent to a cylindrical conduit or tube of standardsize (e.g., the air intake assembly 25, although non-cylindrical inshape, functions equivalently to a standard 4 inch inner diameter PVCconduit). Thus, the air intake assembly 25 achieves the same hydraulicperformance as a standard cylindrical conduit, but with a moreaesthetically pleasing non-cylindrical shape. The interior space 290defines a minor dimension and a major dimension (both perpendicular tothe longitudinal extent and each perpendicular to the other). The majordimension may be, for example, at least twice the minor dimension. It ispossible to construct the intake assembly 25 such that only a portion ofthe interior space 290 is non-cylindrical.

The partition 235 is generally flat and includes a partition window 295.The partition 235 is mounted to one or both of the internal surfaces ofthe first piece 225 and second piece 230. The partition 235 divides theinterior space 290 of the air intake assembly 25 into an inlet side 297and an outlet side 298. The inlet side 297 is above the partition 235and communicates with ambient air (i.e., air surrounding the waterheater 10) through the air inlet 280. The outlet side 298 is below thepartition 235 and communicates with the combustion chamber assembly 20through the elbow 260. The partition 235 reduces or eliminates recyclingof air from the outlet side 298 back into the blower 240. The blower 240is mounted within the inlet side 297 of the interior space 290. Thenon-cylindrical shape of the inlet side 297 accommodates the shape andsize of the blower 240. In the illustrated embodiment, the blower 240 isan axial inflow, centrifugal blower. In the illustrated embodiment, theblower 240 is of relatively small size, producing less than about 15 CFM(“cubic feet per minute”) of airflow with a maximum static pressure heador pressure rating of less than 2 inches water column or in someembodiments a fraction of 1 inch of water column. This is in comparisonto blowers for known power burners which are multiple times larger thanblower 240. For example, in a known residential 120,000 Btu/hr powerburner application, the associated blower has a mid-range operatingairflow rate of about 80 CFM to 140 CFM. Known power burner models ofabove 120,000 Btu/hr employ a blower that operates at 160 CFM and above.Static pressure head of such power burners (i.e., the above-mentioned120,000 Btu/hr power burner and those above 120,000 Btu/hr) is on theorder of 11 inches water column. Commercial water heater power burnershave even higher airflow rates and static pressure ratings than themodels described above.

In other embodiments, the blower 240 could be an axial fan or other airmoving device that achieves the basic functionality of sucking ambientair into the inlet side 297 through the air inlet 280 and forcing theair into the outlet side 298 and combustion chamber assembly 20 atpressures above atmospheric pressure. The term “pressurized,” as usedthroughout this disclosure means at a pressure higher than atmosphericpressure. The blades of the blower 240 are of a known design referred toas a “squirrel cage,” and the blower 240 includes a volute casing 300.The pressure of the air rises as it is forced through the volute casing300. The blower inlet communicates with the air inlet 280 and the bloweroutlet communicates with the partition window 295. A blower wire 305 orpower cord communicates between the pressure sensor 125 and the blower240 and is received in the power cord channel 255.

With reference to FIGS. 3 and 7, the blower 240 sucks ambient airthrough the air inlet 280 into the inlet side 297 at atmosphericpressure, raises the pressure of the air above atmospheric, and forcesthe pressurized air into the outlet side 298 through the partitionwindow 295. In this regard, the inlet side 297 may be referred to as thelow pressure side of the interior space 290 and the outlet side 298 maybe referred to as the high pressure side. Also, the illustrated waterheater 10 may be termed a forced draft water heater because combustionair is pressurized in the combustion chamber, and products of combustionare forced up the flue 40 under the influence of positive pressure atthe air inlet assembly. This is contrasted with an induced draft waterheater 10 in which a blower at the top of the flue 40 draws the productsof combustion up the flue 40 by creating a low pressure region at thetop of the flue 40 (i.e., at the inlet to the blower) and a highpressure region at the outlet of the blower.

Turning now to FIGS. 8-10, the combustion chamber assembly 20 includes astand 310, a bottom plate 315, a skirt 320, a burner-door assembly 325,an air diverter 330, an air distributor plate 335, a primary air pan340, and a condensation tray or condensation pan 345. The bottom plate315 rests on the stand 310 and is joined to the skirt 320 by a foldingor other metal joining method. The skirt 320 extends upwardly from thebottom plate 315, and supports the bottom of the water tank 15. Acombustion chamber 347 (FIG. 10) is defined by the bottom plate 315,skirt 320, and bottom of the tank 15. In the illustrated embodiment, thecombustion chamber 347 is below the water in the water tank 15, and isnot a submerged combustion chamber which is surrounded by water. Thestand 310 elevates the combustion chamber assembly 20 from the floor orsurface on which the water heater 10 stands, to reduce temperatures towhich the floor is exposed during operation of the water heater 10.

The skirt 320 includes an opening 350 and an inlet fitting 352. An inletgasket 353 fits snuggly over the inlet fitting 352, and also fitssnuggly within the elbow 260 of the air intake assembly 25. The inletgasket 353 creates a substantially airtight seal between the air intakeassembly 25 and the combustion chamber assembly 20 so that substantiallyall high pressure air flowing from the air intake assembly 25 isdelivered to the combustion chamber assembly 20 and does not leak to thesurrounding environment. The air inlet 280 is above the combustionchamber 347. The air intake assembly 25 functions as a conduit betweenthe air inlet 280 and the combustion chamber 347.

The burner-door assembly 325 includes a door 355 that fits over theopening 350 in the skirt 320. In the final assembly, a shield 357(FIG. 1) is mounted to the jacket 35 and covers the portions of theburner-door assembly 325 that are exterior of the skirt 320. Theburner-door assembly 325 also includes a gas manifold 360 (FIG. 9)attached to the door 355 and communicating with the gas supply line 205through the door 355. The igniter wire 180 and flame sensor wire 185pass through the door 355 and are surrounded by a grommet 363 or thelike for a substantially air-tight seal between the wires and the door355. In other embodiments, the burner-door assembly 325 may be of thetype described and illustrated in U.S. patent application Ser. No.12/431,525 filed Apr. 28, 2009, the entire contents of which isincorporated into this disclosure by reference.

The burner-door assembly 325 also includes an air duct 365 supported bythe gas manifold 360, a burner 370 supported by the air duct 365, and amounting bracket 375 on the gas manifold 360. Gaseous fuel from the gasmanifold 360 is mixed with primary air in the air duct 365 to form apartially premixed combustible mixture, and the combustible mixture isburned by the burner 370, as will be discussed in more detail below. Themounting bracket 375 supports the hot surface igniter 135 and flamesensor 140 near the burner 370 so that the combustible mixture can beignited and monitored.

The burner 370 includes a condensation drain hole 380. Condensate thatpools on the burner 370 drains through the condensate drain hole 380 tothe condensation pan 345. This can occur, for example, during a coldstart of the water heater 10. Condensation that collects in thecondensation pan 345 evaporates and is exhausted through the flue 40with products of combustion when the water heater 10 is operating atsteady state. More specifically, the condensation pan 345 is dimensionedto cause a sufficient surface area of condensate in the condensation pan345 to be exposed to heat in the combustion chamber 347 to result intotal evaporation of the condensate upon the water heater reachingsteady-steate combustion and normal operation. The condensation pan 345should be of a size sufficient to pass heavy condensation tests.

The air distributor plate 335 is designed to evenly distribute secondaryair in the combustion chamber 347 to promote even combustion at theburner 370. The air distributor plate 335 includes a top surface 410 anda bottom surface 415 that are generally planar and horizontal. In theillustrated embodiment, the air distributor plate 335 includes a firstedge 421, a second edge 422, a third edge 423, a fourth edge 424, afifth edge 425, and a sixth edge 426, all of equal length and at equalangles, and in this regard may be termed a “hex plate” due to itshexagonal shape. Depending from the first edge 421, second edge 422,third edge 423, fourth edge 424, fifth edge 425, and sixth edge 426 is arespective first side wall 431, second side wall 432, third side wall433, fourth side wall 434, fifth side wall 435, and sixth side wall 436.These side walls 431-436 define generally vertical surfaces. Formed inthese respective side walls are a first opening 441, second opening 442,third opening 443, fourth opening 444, fifth opening 445, and sixthopening 446. The first-fifth openings 441-445 may collectively bereferred to as “secondary air openings” and are at least partiallydefined by the generally vertical surfaces.

The first side wall 431 includes a portion that extends from the firstedge 421 to the bottom plate 315, and another portion that extends onlypartially (e.g., halfway) from the first edge 421 toward the bottomplate 315. The first side wall 431 does not extend the entire length ofthe first edge 421. Consequently, the first opening 441 includes aportion that extends fully between the first edge 421 to the bottomplate 315, and another portion that extends only partially from thebottom plate 315 toward the first edge 421 (i.e., the space between thebottom plate 315 and the short portion of the first side wall 431).

The third side wall 433 is actually divided into roughly equal portionson either side of the third opening 443, such that the third opening 443is roughly centered with respect to the third edge 423. The fifthopening 445 is defined at opposite ends of the fifth side wall 435. Thesixth side wall 436 is actually two relatively small wall portions ortabs at opposite ends of the sixth edge 426, such that the sixth opening446 is relatively large and centered with respect to the sixth edge 426.

The air diverter 330 includes a lip 450 and feet 455. The feet 455 siton and are mounted with fasteners to the bottom plate 315, and the lip450 extends along the top surface 410 of the air distributor plate 335along the sixth edge 426 over the sixth opening 446. Side walls of theair diverter 330 extending from opposite ends of the lip 450 down to thefeet 455 are positioned along the sixth side wall 436 portions atopposite ends of the sixth opening 446. Consequently, the air diverter330 surrounds the sixth opening 446, and places the inlet fitting 352and sixth opening 446 in communication. Substantially all combustion airflowing at elevated pressure from the air intake assembly 25 is diverteddownwardly by the air diverter 330, through the sixth opening 446 andunder the air distributor plate 335.

Formed in the air distributor plate 335 are a three slots 460 thataccept three tabs 465 of the primary air pan 340. The tabs 465 areextended up through the slots 460 and bent over to secure the primaryair pan 340 to the bottom surface 415 of the air distributor plate 335.The air distributor plate 335 is mounted to the bottom plate 315 withthree fasteners 470. A primary air plenum is defined between the primaryair pan 340 and the bottom surface 415 of the air distributor plate 335,and a secondary air plenum or secondary air distribution space isdefined by the space that surrounds the primary air pan 340 between thebottom surface 415 of the air distributor plate 335 and the bottom plate315.

The air distributor plate 335 also includes an integral first locatingmember 485, an integral burner locating member 490, an integral manifoldpocket 495, and an integral manifold clearance indentation 500. Thefirst locating member 485 is in the form of a concave bump or boss inthe top surface 410 of the air distributor plate 335. The first locatingmember 485 and integral manifold pocket 495 define two clocking pointsfor mounting the condensation pan 345 to the air distributor plate 335.More specifically, the condensation pan 345 includes a first matingportion and a second mating portion that receive the respective firstlocating member 485 and the integral manifold pocket 495. This ensuresthat the condensation pan 345 is positioned properly to receivecondensation that drains from the burner 370. With the first locatingmember and integral manifold pocket 495 received in indentations in thecondensation pan 345, a single threaded fastener 510 may be used tosecure the condensation pan 345 to the air distributor plate 335.

The burner locating member 490 is in the form of a raised trapezoidalbase in the top surface 410 and a primary air hole 515. The a primaryair opening or primary air hole 515 communicates with the primary airplenum. The air duct 365 of the burner-door assembly 325 fits snugglyaround the raised trapezoidal base of the burner 370 locating member,such that substantially all primary air flowing through the primary airhole 515 from the primary air plenum flows into the air duct 365 foreventual delivery to the burner 370. In other embodiments, the base ofthe burner locating member 490 can be other shapes, but it preferablywill snuggly receive the bottom edge of the air duct 365 to ensure thatthe air duct 365 receives substantially all primary air flowing out ofthe primary air plenum and primary air hole 515.

The manifold pocket 495 is a convex (with respect to the top surface410) deformation with an opening 520 at one end. The manifold pocket 495receives a distal end of the gas manifold 360 to secure the manifoldwith respect to the combustion chamber 347. The manifold clearanceindentation 500 is a concave (with respect to the top surface 410)deformation that permits the burner-door assembly 325 to be insertedinto the opening 350 in the skirt 320 at an angle and then tilted intothe operable position without the gas manifold 360 bumping into the topsurface 410 of the air distributor plate 335.

In operation, the controller 105 monitors water temperature with thetemperature probe 190 and controls the temperature of water within thetank 15 based on the settings input by an operator through the userinterface 120. When water temperature drops below a low-end set point(e.g., due to standby heat loss or during a draw of hot water from thetank 15 and the resultant introduction of cold water into the tank 15through the dip tube 90), the controller 105 engages the blower 240.When operating properly, the blower 240 creates high pressure in theoutlet side 298 of the air intake assembly 25. The high pressure issensed by the pressure sensor 125 through the pressure tap 126, and asignal is sent to the controller 105 confirming that the blower 240 isoperating properly.

The high pressure air from the air intake assembly 25 is forced into thecombustion assembly through the inlet fitting 3352 on the skirt 320. Theair diverter 330 directs the high pressure air into the primary airplenum and secondary air plenum under the air distributor plate 335. Theflow of high pressure air into the primary air plenum is subsonic, whichresults in feedback waves through the air particles to the air diverter330. The feedback waves result in a balance of high pressure air flowinginto the primary air plenum and secondary air plenum, and avoid anundesirable amount of high pressure air flowing into the primary airplenum at the expense of air supply to the secondary air plenum. Thepressurized primary air flows from the primary air plenum up through theprimary air hole 515, and into the air duct 365 of the burner-doorassembly 325.

Once the controller 105 has confirmed that the blower 240 is operatingproperly, the controller 105 provides power (i.e., electrical current)to the hot surface igniter 135, to cause the hot surface igniter 135 toachieve a temperature sufficient to ignite a fuel-air mixture. Thecontroller 105 determines that the hot surface igniter 135 has achievedsuch temperature by known means, or assumes that the hot surface igniter135 has achieved such temperature after the passage of sufficient time.

Once the controller 105 has confirmed that the blower 240 is operatingproperly and the hot surface igniter 135 is at an appropriatetemperature to ignite a fuel-air mixture, the controller 105 opens thegas valve 115 to permit gaseous fuel to flow from the gas hook-up line200, through the gas valve 115, to the gas supply line 205, and to thegas manifold 360. The gaseous fuel flows out of the gas manifold 360into the air duct 365, where it mixes with pressurized primary air tocreate a pressurized (above atmospheric pressure) partially premixedfuel-air mixture. The fuel-air mixture flows from the air duct 365 intothe burner 370.

The illustrated burner 370 is a pancake style burner 370 having burnerorifices around its perimeter. The illustrated burner 370 is in thecombustion chamber 347 and is below the water tank 15, and may be termedan “upwardly firing” burner because products of combustion rise upwardlyfrom the burner 370. The fuel-air mixture flows out of the burnerorifices and is ignited by the hot surface igniter 135 to create a ringof flame around the burner 370. The flame sensor 140 confirms to thecontroller 105 that the flame is present on the burner 370. The flamecreated by the burner 370 combusting the primary air and fuel mixture isa partially premixed but substantially diffusion flame having anenvelope. Combustion of the diffusion flame is completed within thediffusion flame envelope in the presence of secondary air.

The partially premixed, diffusion flame produced by the burner 370 inthe present invention includes a fuel-rich core that is surrounded by aflame envelope into which secondary air is diffused to completecombustion and lower NOx emissions. The core region includesinsufficient air to complete combustion of the fuel, which is why thediffusion flame is referred to as “partially” premixed. The diffusionflame front consequently propagates from the flame envelope inward tocomplete the combustion of the core region with the help of thesecondary air. The majority of air required for complete combustion andreduced NOx is added at the flame envelope in the form of secondary air.

The partially premixed diffusion flame of the present invention isdistinguished from the flame created by fully premixed power burners.Fully premixed power burners include sufficient air in the air/fuelpremixture to support full combustion of the fuel and low NOx emissions.Indeed, in most power burner applications, the blower that is part ofthe power burner is pushing more air than is required for completecombustion because the blower is also required to force the products ofcombustion out the flue at elevated pressure for the purpose of directventing or utilization of the products of combustion for another purpose(e.g., space heating) downstream of the water heater. The blower in apower burner is typically much larger than the blower contemplated bythe present invention, in terms of airflow (measured in CFM) and staticpressure head (measured in inches of water column) as discussed above.

Secondary air collects in the secondary air plenum under the airdistributor plate 335. The secondary air is pressurized (i.e., aboveatmospheric pressure) and flows out of the secondary air plenum throughthe first opening 441, second opening 442, third opening 443, fourthopening 444, and fifth opening 445. The openings 441-445 are sized andspaced to create substantially axisymmetric distribution of secondaryair flowing out of the secondary air plenum and into the combustionchamber 347 around the burner 370. The air distributor plate 335 andopenings 441-445 create a substantially uniform supply of secondary airto the combustion chamber 347. Uniform airflow out of the secondary airplenum reduces potential surface/floor temperature issues and also givesrise to improved combustion. A design that minimizes standard deviationof air flow rates from the average secondary air flow rate for theopenings 441-445 is desirable for improving combustion. The combinedsize and geometry of the primary plenum and air distributor plate 335can achieve standard deviations of less than 1.35, with standarddeviations reaching 0.88 and as low as 0.08 in some embodiments.

To summarize the air flow and combustion process, pressurized combustionair flows under the air distributor plate 335 from the air diverter 330through the sixth opening 446 at one side of the air distributor plate335. The pressurized combustion air is divided into primary air, whichflows into the primary air plenum, and secondary air, which flows intothe secondary air plenum. Both the primary air and secondary air arepressurized due to the blower 240 forcing the air into the combustionchamber 347. The primary air is mixed with gaseous fuel and is ignitedat the perimeter of the burner 370 to create a diffusion flame. Thesecondary air, despite entering the secondary air plenum from the sixthside of the air distributor plate 335, is substantially evenlydistributed through the first through fifth openings 441-445 due to thesize, shape, and position of the openings 441-445. The high pressuresecondary air flows around the sides of the air distributor plate 335and completes combustion of the fuel-air mixture within the envelope ofthe diffusion flame.

Combustion of the fuel-air mixture at the diffusion flame createsproducts of combustion. The blower 240 pressurizes the entire combustionchamber 347 to a pressure higher than atmospheric, and the products ofcombustion also have natural buoyancy owing to their high temperature.As a result, the products of combustion rise and are forced into theflue 40. The products of combustion transfer heat to the baffle 45 andflue 40, which in turn transfer heat to the water. The burner 370continues to generate products of combustion as discussed above, untilthe temperature probe 190 senses that the water temperature has reachedthe desired set point or high-end set point (as programmed at the userinterface 120).

The flow of the products of combustion is restricted as they flow upthrough the flue 40 by the restricted flow path caused by the baffle 45.The products of combustion lose pressure as they flow from the flueinlet end (lower end of the flue 40 communicating with the combustionchamber 347) to the flue outlet end (upper end of the flue 40communicating with the venting structure 77). The blower 240 is sized tocreate a known pressure (also called head pressure or head) in thecombustion chamber 347. Given the known pressure in the combustionchamber 347, the flue 40 and baffle 45 are designed to reduce pressurein the products of combustion to near atmospheric at the flue outlet topermit the water heater 10 to benefit from a pressurized diffusion flamein the combustion chamber 347, a restricted flow flue 40 and baffle 45assembly to increase dwell time of the products of combustion, and aCategory I atmospheric venting configuration.

The FV sensor 130 is positioned external of the combustion chamberassembly 20, relatively low or close to ground level because flammablevapors tend to be heavier than air and would typically collect close toground level. The FV sensor 130 is lower than the air inlet 280, and inthe illustrated embodiment is lower than the burner 370 and combustionchamber 347. If during the operation of the water heater 10, the FVsensor 130 senses the presence of flammable vapors outside of the waterheater 10 in concentrations above a maximum threshold, the FV sensor 130generates a signal to the controller 105 and the controller 105 can shutdown operation of the water heater 10 by closing the gas valve 115.Elevating the air inlet with respect to the FV sensor 130 increases thelikelihood that the FV sensor 130 will sense the presence of theflammable vapors and the controller 105 will shut down the gas valve 115prior to the flammable vapors being entrained in the incoming combustionair and reaching the burner 370. In view of the FV sensor 130 and thefunctionality described above, the water heater 10 of the presentinvention is deemed a flammable vapor ignition resistant (“FVIR”) waterheater.

Similarly, if the pressure sensor 125 senses that the pressure of airdownstream of the blower 240 (i.e., in the outlet side 298) is below aminimum threshold, the pressure sensor 125 generates a signal to thecontroller 105 and the controller 105 can shut down operation of thewater heater 10 by closing the gas valve 115. Likewise, if the flamesensor 140 fails to sense the presence of a flame at the burner 370, theflame sensor 140 generates a signal to the controller 105 to shut downoperation of the water heater 10. In all cases, the generation of asignal can include the cessation of a signal or the changing of asignal.

Thus, the invention provides, among other things, a water heaterincluding a 24 volt controller to control various powered aspects of thewater heater, a water heater, a sealed combustion chamber water heaterwith a FV sensor lower than the air inlet for the combustion chamber; aforced-draft water heater having a blower in the air inlet and a bafflein the flue such that combustion occurs at elevated pressure butpressure of products of combustion drop to near atmospheric at theoutlet end of the flue; a water heater having an air intake assemblyhaving an internally-mounted blower mounted inside; and a water heaterhaving an air distributor plate for substantially axisymmetricdistribution of secondary air in the combustion chamber. Variousfeatures and advantages of the invention are set forth in the followingclaims.

1. A water heater comprising: a tank for water to be heated; a poweredanode extending into the tank and creating an electrical current toreduce corrosion of the tank; a combustion chamber; an exhauststructure; a flue in the tank communicating between the combustionchamber and the exhaust structure; a burner in the combustion chamberoperable to burn a mixture of primary combustion air with gaseous fuelin a partially premixed but substantially diffusion flame having anenvelope, such that combustion of the mixture is completed at thediffusion flame envelope in the presence of secondary air to produceproducts of combustion, the products of combustion flowing through theflue to the exhaust structure to heat the water in the tank; acentrifugal blower forcing primary and secondary air into the combustionchamber at pressure above atmospheric; a gas valve for controlling asupply of gaseous fuel to the burner; a user interface for programmingoperating parameters of the water heater; and a 24 V controller thatprovides power to the user interface and powered anode, and thatcontrols operation of the blower and gas valve.
 2. The water heater ofclaim 1, further comprising a pressure sensor operatively interconnectedwith the 24 V controller; wherein the pressure sensor generates a signalin response to sensing that the pressure of air downstream of the bloweris below a minimum threshold; and wherein the 24 V controller closes thegas valve in response to receiving the signal from the pressure sensor.3. The water heater of claim 1, further comprising a flammable vaporsensor operatively interconnected with the 24 V controller; wherein theflammable vapor sensor generates a signal in response to sensing thepresence of flammable vapors outside of the water heater inconcentrations above a maximum threshold; and wherein the 24 Vcontroller closes the gas valve in response to receiving the signal fromthe flammable vapor sensor.
 4. The water heater of claim 1, furthercomprising a pressure sensor sensing the pressure of air downstream ofthe blower; and a flammable vapor sensor sensing the presence offlammable vapors outside of the water heater; wherein the 24 Vcontroller closes the gas valve upon the occurrence of any of thefollowing: (a) conditions dictated by the user interface, (b) thepressure sensor sensing air pressure below a minimum threshold, and (c)the flammable vapor sensor sensing flammable vapors external to thewater heater in concentrations above a maximum threshold.
 5. A waterheater comprising: a tank for water to be heated; a combustion chamber;an exhaust structure; a flue in the tank communicating between thecombustion chamber and the exhaust structure; a burner in the combustionchamber operable to burn a mixture of primary combustion air withgaseous fuel in a partially premixed but substantially diffusion flamehaving an envelope, such that combustion of the mixture is completed atthe diffusion flame envelope in the presence of secondary air to produceproducts of combustion, the products of combustion flowing through theflue to the exhaust structure to heat the water in the tank; an airintake assembly including an air inlet above the combustion chamber anda conduit communicating between the air inlet and the combustionchamber; a centrifugal blower within the air intake assembly operable tosuck air into the air intake assembly through the air inlet and forceprimary and secondary air into the combustion chamber through theconduit at pressure above atmospheric; a gas valve for controlling asupply of gaseous fuel to the burner; a controller that controlsoperation of the blower and gas valve; and a flammable vapor sensorexternal of the combustion chamber and lower than the air inlet, theflammable vapor sensor being operatively interconnected with thecontroller and operable to generate a signal in response to sensing thepresence of flammable vapors outside of the water heater inconcentrations above a maximum threshold; wherein all primary andsecondary combustion air supplied to the combustion chamber flowsthrough the air inlet and conduit; and wherein the controller closes thegas valve in response to receiving the signal from the flammable vaporsensor.
 6. The water heater of claim 5, wherein the flammable vaporsensor is lower than at least one of the combustion chamber and burner.7. A water heater comprising: a tank for water to be heated; acombustion chamber; an exhaust structure; a flue in the tankcommunicating between the combustion chamber and the exhaust structure;a burner in the combustion chamber operable to burn a mixture of primarycombustion air with gaseous fuel in a partially premixed butsubstantially diffusion flame having an envelope, such that combustionof the mixture is completed at the diffusion flame envelope in thepresence of secondary air to produce products of combustion, theproducts of combustion flowing through the flue to the exhaust structureto heat the water in the tank; an air intake assembly including an airinlet above the combustion chamber and a conduit communicating betweenthe air inlet and the combustion chamber; a centrifugal blower withinthe air intake assembly operable to suck air into the air intakeassembly through the air inlet and force primary and secondary air intothe combustion chamber through the conduit at pressure aboveatmospheric; a gas valve for controlling a supply of gaseous fuel to theburner; a controller that controls operation of the blower and gasvalve; and a baffle in the flue that restricts flow sufficiently toreduce the pressure of the products of combustion to near atmosphericupon flowing out of the flue into the exhaust structure.
 8. The waterheater of claim 7, wherein the blower is an axial-intake, centrifugalblower.
 9. The water heater of claim 7, wherein the exhaust structure isa category I vent structure.
 10. A water heater comprising: a tank forwater to be heated; a combustion chamber; an exhaust structure; a fluein the tank communicating between the combustion chamber and the exhauststructure; a burner in the combustion chamber operable to burn a mixtureof primary combustion air with gaseous fuel in a partially premixed butsubstantially diffusion flame having an envelope, such that combustionof the mixture is completed at the diffusion flame envelope in thepresence of secondary air to produce products of combustion, theproducts of combustion flowing through the flue to the exhaust structureto heat the water in the tank; an air intake assembly including an airinlet above the combustion chamber and a conduit communicating betweenthe air inlet and the combustion chamber; a centrifugal blower withinthe air intake assembly operable to suck air into the air intakeassembly through the air inlet and force primary and secondary air intothe combustion chamber through the conduit at pressure aboveatmospheric; a gas valve for controlling a supply of gaseous fuel to theburner; and a controller that controls operation of the blower and gasvalve; wherein the air intake assembly includes an interior space, allprimary and secondary air being provided to the combustion chamberflowing through the interior space; and wherein the blower is mountedwithin the interior space of the air intake assembly.
 11. The waterheater of claim 10, wherein the blower is an axial-intake, centrifugalblower.
 12. The water heater of claim 10, wherein the air intakeassembly includes a longitudinal extent; and wherein the air intakeassembly includes a two-piece construction divided along thelongitudinal extent of the air intake assembly.
 13. The water heater ofclaim 10, wherein the interior space of the air intake assembly isnon-cylindrical and has an equivalent hydraulic diameter of a four inchinner diameter tube.
 14. The water heater of claim 10, wherein at leasta portion of the interior space of the air intake assembly isnon-cylindrical to accommodate mounting the blower in the interiorspace; and wherein the non-cylindrical portion of the interior spacedefines a minor dimension and a major dimension that is perpendicular tothe minor dimension and at least twice the minor dimension.
 15. Thewater heater of claim 10, wherein the air intake assembly includes apartition that divides the interior space into an inlet sidecommunicating with ambient air and an outlet side communicating with thecombustion chamber, the partition including a window; and wherein theblower is within the inlet side of the interior space and forces primaryand secondary air into the outlet side of the interior space through thewindow in the partition.
 16. The water heater of claim 15, wherein theair intake assembly includes a louvered opening communicating betweenthe inlet side and ambient air; and wherein all air sucked into theinlet side of the interior space by the blower flows through thelouvered opening.
 17. The water heater of claim 10, further comprising apressure sensor communicating with the interior space, the pressuresensor operable to disable the gas valve to cut off the supply ofgaseous fuel to the burner when pressure within the interior space dropsbelow a minimum threshold.
 18. The water heater of claim 17, wherein theair intake assembly includes an exterior surface, a sensor mountingcavity in the exterior surface and a wire routing channel in theexterior surface; wherein the air intake assembly further includes ahole communicating between the sensor mounting cavity and the interiorspace; wherein the pressure sensor is mounted within the sensor mountingcavity and communicates with the interior space through the hole; andwherein the blower includes a power cord that is received in the wirerouting channel.
 19. A water heater comprising: a tank for water to beheated; a combustion chamber; an exhaust structure; a flue in the tankcommunicating between the combustion chamber and the exhaust structure;a burner in the combustion chamber operable to burn a mixture of primarycombustion air with gaseous fuel in a partially premixed butsubstantially diffusion flame having an envelope, such that combustionof the mixture is completed at the diffusion flame envelope in thepresence of secondary air to produce products of combustion, theproducts of combustion flowing through the flue to the exhaust structureto heat the water in the tank; an air intake assembly including an airinlet above the combustion chamber and a conduit communicating betweenthe air inlet and the combustion chamber; a centrifugal blower withinthe air intake assembly operable to suck air into the air intakeassembly through the air inlet and force primary and secondary air intothe combustion chamber through the conduit at pressure aboveatmospheric; a gas valve for controlling a supply of gaseous fuel to theburner; a controller that controls operation of the blower and gasvalve; an air distributor plate having a generally horizontal topsurface defining a plurality of edges, and a bottom surface at leastpartially defining a secondary air distribution space and a primary airplenum; wherein all primary and secondary air flows into the respectiveprimary air plenum and secondary air distribution space; wherein the airdistributor plate defines a primary air opening for the provision ofprimary air from the primary air plenum to the burner; and wherein theair distributor plate includes a plurality of secondary air openings forsubstantially axisymmetric distribution of secondary air around the airdistributor plate.
 20. The water heater of claim 19, wherein the airdistributor plate has first, second, third, fourth, fifth, and sixthedges; wherein the air distributor plate has generally vertical surfacesextending from the first, second, third, fourth, and fifth edges to atleast partially define the secondary air distribution space; wherein thegenerally vertical surfaces define the secondary air openings; andwherein the substantially axisymmetric distribution includes secondaryair flow rates through the plurality secondary air openings withstandard deviation less than 0.88 from average secondary air flow rate.21. The water heater of claim 20, wherein the standard deviation is lessthan 0.10.
 22. The water heater of claim 20, wherein the standarddeviation is about 0.08.
 23. The water heater of claim 19, furthercomprising an air diverter secured to an edge of the air distributorplate to direct all primary and secondary combustion air from an airintake to the respective primary air plenum and secondary airdistribution space.
 24. The water heater of claim 19, further comprisinga plenum pan mounted to a bottom surface of air distributor plate to atleast partially define the primary air plenum.
 25. The water heater ofclaim 24, wherein the plenum pan includes a plurality of tabs; whereinthe distributor plate includes a plurality of slots through which tabsextend; and wherein the tabs are bent against the top surface ofdistributor plate to secure the plenum pan to the bottom surface of thedistributor plate.
 26. The water heater of claim 19, wherein thedistributor plate includes an integral first locating member, anintegral burner locating member, an integral manifold pocket, and anintegral manifold clearance indentation; wherein a portion of the burnermates with the burner locating member; the water heater furthercomprising a gas manifold for the provision of gaseous fuel from the gasvalve to the burner, the manifold extending into the manifold clearanceindentation during installation, and the manifold extending into themanifold pocket when installed; and a condensation tray including afirst mating portion that mates with the first locating member, and asecond mating portion that extends around the manifold pocket.
 27. Thewater heater of claim 19, wherein the distributor plate includes firstand second locating members; the water heater further comprising acondensation tray having first and second clocking points that mate withthe respective first and second locating members; wherein thecondensation tray is secured to the distributor plate with a singlethreaded fastener in combination with mating of the first and secondclocking points with the first and second locating members.
 28. Thewater heater of claim 19, further comprising a condensation tray mountedin the combustion chamber; wherein the burner includes a condensatedrain that directs condensation to the condensation tray.
 29. The waterheater of claim 28, wherein the condensation tray has a containmentcapacity at least equal to the volume of condensate predicted duringheavy condensation cold start of the water heater.
 30. The water heaterof claim 28, wherein the condensation tray is dimensioned to cause asufficient surface area of condensate in the condensation tray to beexposed to heat in the combustion chamber to result in total evaporationof the condensate upon the water heater reaching steady-state combustionand normal operation.